The disclosed invention relates to an automatic gain control amplifier which is particularly advantageous for use with voice detection systems.
Voice detection systems are generally utilized in situations where the presence or absence of speech is used to initiate a defined procedure. For example, a voice detection system can be used in a phone answering system to control the recording of the incoming call when speech is detected or the playback of a recorded message when a predetermined pause in speech is detected.
Amplifiers having automatic gain control are generally used in such systems for providing a reasonably constant amplifier output which is representative of the sound input to the voice detection system. The amplifier output is analyzed to determine whether speech is present at the input to the system.
However, prior art automatic gain control amplifiers have made voice detection systems susceptible to producing erroneous speech-present outputs caused by background noise present at the source of the voice signals. Such erroneous detection is generally caused by the tendency of prior art automatic gain control amplifiers to increase in gain during time periods when no speech is being inputted to the system. When the gain increases sufficiently, background noise appears as the nominal input to the system which causes an amplifier output level comparable to the output when speech is actually present.
It is therefore an object of the invention to provide an improved automatic gain control amplifier in which gain control is not affected by input levels below a predetermined threshold.
Another object of the invention is to provide an improved automatic gain control amplifier in which gain is increased when the amplifier output is below a fixed threshold and above a controlled variable reference.
Still another object of the invention is to provide an improved automatic gain control which is useful in voice detection systems and allows automatic gain control only when speech is detected.
Another object of the invention is to provide an improved automatic gain control amplifier for use in voice detection systems which is not susceptible to an increase in gain when the input level is reduced to a level below that representative of speech.
A further object of the invention is to provide an improved automatic gain control for use in a voice detection system and wherein gain recovery rate is controlled by the speech input to the voice detection system.